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The influence of graphite morphology on the elastic behaviour of cast irons
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.
2005 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The morphology of the graphite phase largely controls the resulting properties of different grades of cast iron. In flake graphite cast irons, the mechanical properties are low while the thermal conductivity is high. In contrast, with spheroidal graphite cast irons, the mechanical properties are high while the thermal conductivity is low. These differences must be accounted for in the design work of diesel engines when choosing the material grade of different components. In this work the influence of graphite morphology on the deformation behaviour of cast irons has been studied with an emphasis on the elastic region.

The studied material grades originated from castings for marine diesel engine piston rings with different chemical analyses. This gave rise to six material grades with different nodularities and three material grades with different carbon equivalents. For these different material grades the mechanical properties were determined and the deformation behaviour was examined by means of mechanical testing and acoustic emission measurements. These data were correlated to microstructural parameters.

When studying the modulus of elasticity it was found that the modulus of elasticity of the inherent graphite phase depends on the roundness of the graphite particles and is due to the different elasticity in the two main crystallographic directions of the graphite lattice. The graphite particles are structured in different ways in flake graphite and spheroidal graphite, which gives rise to different stiffness at loading. The modelling and experiments support the observation that the average modulus of elasticity of the graphite phase depends on the morphology. This improves the understanding of the differences in the modulus of elasticity of the different studied material grades. The correlation between the modulus of elasticity of the graphite phase and the bulk nodularity of the cast iron specimen was found to be linear. This linear function was used when modelling the effective modulus of elasticity of the different cast iron grades with very high accuracy between experimental and theoretical values.

Another factor affecting the elastic response when subjecting a cast iron component to tensile load was found to be the plastic deformation that actually occurs at very low strains for all of the studied cast iron grades. lt was found that the plastic deformation in the low strain, elastic region governs the modulus of elasticity of cast irons. The plastic deformation in the seemingly elastic region was quantified by using acoustic emission measurements. These measurements showed that the amount of plastic deformation in the elastic region was largely controlled by the graphite morphology. It was concluded that, as the roundness of the graphite particles increases, the plastic activity in the elastic region decreases and the energy absorption ability increases.

Place, publisher, year, edition, pages
2005. , 25 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1140
Keyword [en]
Cast iron, flake graphite, compacted graphite, spheroidal graphite, elastic behaviour, modulus of elasticity, graphite modulus of elasticity, nodularity, aspect ratio, plastic deformation, acoustic emission
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-30053Local ID: 15513ISBN: 91-8529-707-0 (print)OAI: oai:DiVA.org:liu-30053DiVA: diva2:250874
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-11-28
List of papers
1. High Performance Piston Rings for Two-Stroke Marine Diesel Engines
Open this publication in new window or tab >>High Performance Piston Rings for Two-Stroke Marine Diesel Engines
2004 (English)In: Proceedings // 24th CIMAC World Congress on Combustion Engine Technology, 2004Conference paper, Published paper (Other academic)
Abstract [en]

The piston ring is a key component in a marine combustion engine. High mechanical loads, relatively high temperatures and corrosive gases and liquids influence its performance in terms of sealing capacity, wear of cylinder liner and the ring itself. Base material of the ring, coating technology and ring geometry design are discussed in the article.

When tailoring cast iron materials suitable as piston ring base material two parameters are of importance; the morphology of the graphite and the constituents of the matrix. To optimize the properties of the cast iron a compromise is needed to achieve a satisfactory performance of the piston rings.

Daros Piston Rings AB is currently developing a second generation of chromium-ceramic coating the so called Z-chrome. The objective of this project has been to increase the maximum operating temperature of the coating and leave the other characteristics of the coating unaffected. The difference between the commercial Daros coating Tritor® and the Z-chrome is the ceramic component included in the coated layer.

Insufficient conformability of piston ring and liner geometry may produce a large local cylinder wall pressure which will destroy the oil film leading to uncontrolled wear and scuffing. Lack of conformability can also produce leakage paths for the combustion gases. Therefore a correct ring shape is of utmost importance. A new design philosophy designated OPCORE® has been developed and is presented here.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-14415 (URN)
Conference
24th CIMAC World Congress on Combustion Engine Technology, June 7-11, Kyoto, Japan
Available from: 2007-04-20 Created: 2007-04-20 Last updated: 2013-11-28
2. Comparison of Mechanical Properties in Flake Graphite and Compacted Graphite Cast Irons for Piston Rings
Open this publication in new window or tab >>Comparison of Mechanical Properties in Flake Graphite and Compacted Graphite Cast Irons for Piston Rings
2004 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, Vol. 17, no 2, 65-71 p.Article in journal (Refereed) Published
Abstract [en]

Cast iron is often used as the piston ring material in marine diesel engines. The mechanical properties of cast irons are to a great extent governed by the size, distribution and shape of the incorporated graphite particles. In a set of experiments, the mechanical properties of a pearlitic grey cast iron and a pearlitic compacted graphite cast iron are compared. Both cast iron grades have a eutectic composition. The experiments confirm the importance of micro-yielding of the matrix at the tip of the graphite particles on the macroelastic behaviour of the studied cast irons. This applies especially for the flake graphite cast iron where the graphite tip is sharper and the matrix bridges between the graphite particles are shorter than in the case of the compacted graphite cast iron resulting in micro-yielding at the graphite tip at a very low macro-stress and macro-strain. The high local stresses at the graphite tips also result in the opening of the graphite cavities which is much more severe in the flake graphite cast iron than in the compacted graphite cast iron. The mechanical properties of the eutectic flake graphite cast iron are largely affected by the size and amount of the graphite particles. The smaller the graphite particles (faster solidification) in the microstructure, the lower the values of the mechanical properties. In compacted graphite cast iron, the macro-elastic behaviour is influenced by the matrix and the overall coarseness of the microstructure to a greater extent and the effect of the incorporated graphite particle size is much less pronounced.

Keyword
Mechanical properties, Eutectic pearlitic cast iron, Graphite shape, compacted graphite, Flake graphite, Micro-yelding
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-14416 (URN)10.1179/136404604225017474 (DOI)
Available from: 2007-04-20 Created: 2007-04-20 Last updated: 2013-11-28
3. Modelling the Effect of Graphite Morphology on the Modulus of Elasticity in Cast Irons
Open this publication in new window or tab >>Modelling the Effect of Graphite Morphology on the Modulus of Elasticity in Cast Irons
2004 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, Vol. 17, no 5, 271-279 p.Article in journal (Refereed) Published
Abstract [en]

Nine grades of pearlitic cast iron containing different graphite morphologies (from flake, compacted and spheroidal) have been studied. The parameters investigated include the graphite aspect ratio, nodularity, graphite size and modulus of elasticity. These parameters have been correlated and compared with different existing bound and model equations. It has been found that the modulus of elasticity of the graphite phase increases as the aspect ratio and nodularity of the graphite increases, i.e.flake graphite gives a lower modulus of elasticity than spheroidal graphite. The experimental values of the modulus of elasticity show good agreement to bound and model equations, although flake graphite cast irons show higher deviation from the modelled values. An equation for the correlation between the graphite modulus of elasticity and the nodularity is presented. Introducing this linear correlation into an existing model for the determination of the effective modulus of elasticity gives a continuous function, including all grades of cast irons, with a very good agreement with experimental values. The modulus of elasticity of cast irons can be accurately predicted from both bound and especially model equations, using the aspect ratio and nodularity of the contained graphite particles. The fit is improved by using a modulus of elasticity of the graphite phase that is based on the graphite morphology, considering that the modulus of elasticity of the graphite is different in the basal and prismatic planes.

Keyword
Cast iron, Modulus of elasticity, Graphite morphology, Graphite aspect ratio, Compacted graphite, Flake graphite, Spheroidal graphite, Modelling
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-14417 (URN)10.1179/136404604225022694 (DOI)
Available from: 2007-04-20 Created: 2007-04-20 Last updated: 2013-11-28
4. Studying Elastic Deformation Behaviour of Cast Irons by Acoustic Emission
Open this publication in new window or tab >>Studying Elastic Deformation Behaviour of Cast Irons by Acoustic Emission
2005 (English)In: International Journal of Cast Metals Research, ISSN 1364-0461, Vol. 18, no 4, 249-256 p.Article in journal (Refereed) Published
Abstract [en]

The deformation of metallic materials includes both an elastic and a plastic deformation. In the case of cast irons, the elastic region becomes less pronounced as the graphite changes from spheroidal to flake shaped, as observed in nodular and grey cast iron, respectively. The present study aims to correlate the shape of the graphite phase with the deformation behaviour, where the plastic deformation and other strain accommodating events are quantified by measurements of the acoustic emission events occurring in the interior of the material at loading. It also aims to explain how the appearance of cast iron stress–strain curves depends on the graphite morphology where, for instance, spheroidal graphite cast irons exhibit a seemingly linear elastic behaviour in contrast to flake graphite cast irons. The present study includes a series of pearlitic cast iron material grades with differences in nodularity and carbon equivalent, respectively. It is shown that as the roundness of the graphite phase increases, the ability to absorb energy increases. The measured acoustic emission indicates that plastic deformation occurs in the seemingly linear elastic region regardless of the cast iron grade, i.e. no cast iron grade exhibits perfect linear elasticity. The plastic deformation rate in the elastic region increases as the roundness of the graphite decreases and as the carbon equivalent increases. It is shown that the plastic deformation governs the resulting modulus of elasticity in all kind of cast irons, i.e. the modulus of elasticity decreases as the yielding of the material increases. The present study improves the understanding of the deformation behaviour in the elastic region of different cast irons. The survey shows that acoustic emission testing is a useful method when studying the deformation behaviour of cast irons.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-14418 (URN)10.1179/136404605225023117 (DOI)
Available from: 2007-04-20 Created: 2007-04-20 Last updated: 2013-11-28

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